Wireless networks for wide area coverage are typically deployed using multiple geographically separated sites. A site is the physical location where antennas and/or radio communications equipment, e.g. one or more radio base stations, are placed. Each site may comprise several cells, sometimes referred to as sectors. A common scenario is that a base station comprises three antennas facing in different directions, thus creating a three-sector cell. However, a base station may also be connected to one or more remote antennas. Thus, a single base station could serve several cells, which may be geographically distributed. When migrating to newer wireless access technologies, for example updating from a WCDMA (Wideband Code Division Multiple Access) system to LTE (Long Term Evolution), operators will typically aim to reuse as much as possible of the already existing infrastructure in order to reduce costs. In this situation, the location of the new needed sites is more or less predetermined from the locations of the old technology sites.
Furthermore, cells within close proximity of each other are often mutually dependent, in the sense that transmissions in one cell influence ongoing transmissions in neighboring cells by means of inter-cell interference.
Recently, wireless system architectures that allow for coordinated transmission and reception across multiple cells and sites have attracted significant interest. By coordinating the operation of radio base stations that serve mutually dependent cells, it is possible to reduce interference and enhance signal strengths and/or signal quality. One early example of coordinated transmission is soft handover in CDMA (Code Division Multipoint Access) systems, e.g. WCDMA (Wideband CDMA), where downlink transmissions to mobile terminals at the cell border originate from multiple cells. By combining the signals received from the multiple cells, the probability increases that a mobile terminal at the cell border will be able to receive the signal with adequate strength. Similarly, soft handover in the uplink in CDMA is an example of coordinated reception. Uplink soft handover implies that the uplink signal is received at several cells, and later on the information is combined at a central node. Again, by combining signals from several cells, the probability that the signal is correctly received increases.
Nowadays, more advanced and complex forms of coordination across cells are also considered. Some examples are downlink joint coherent processing, and uplink reception across multiple cells and sites using soft values. Different forms of inter-cell interference coordination techniques, like coordinated scheduling and coordinated beam forming, are also of interest. For instance, by using coordinated beam forming, transmission energy from several cells could be directed towards a mobile terminal, thereby improving signal strength. In the present disclosure, the term CoMP, or coordinated multipoint transmission and reception, will be used to refer to all these schemes, as well as other techniques for coordinated transmission/reception.
A group of cells which perform coordinated transmission and/or reception will hereinafter be referred to as a coordination area or a subgroup. Generally speaking, the benefit from using CoMP increases with the size of the coordination area. However, in order to coordinate their operation, all the cells within a coordination area need to exchange information with each other. This information exchange is typically done over a transport network, i.e. a network connecting the radio base stations in the radio access network. For instance, the transport network could use point-to-point microwave connections or fibre optic cables to connect the cells. The need for information exchange between the cells implies that the size of the coordination areas needs to be limited, otherwise the data exchange capabilities in the transport network may be exceeded. Moreover, coordination between a large number of cells will increase computational complexity as well as latency, since the coordination information will need to travel greater distances. Considering that the gains associated with an extended coordination area typically diminish as the size of the coordination area increases, the size of the coordination areas should be chosen to provide a good balance between performance and complexity.
To illustrate the concept of coordination areas, FIG. 1 shows a wireless communications network 100 in the form of an LTE (Long Term Evolution) radio access network, comprising a number of cells 130, 132 . . . 148. Each cell is served by a radio base station; in this particular example an evolved NodeB (eNB). Thus, the eNB that serves a particular cell handles radio communication with mobile terminals that are within the coverage area of the cell. In FIG. 1, cell 136 is served by network node 110, and cell 134 is served by network node 150. As mentioned above, one eNB may serve several cells. For instance, eNB 180 serves both cells 140 and 142 via a remote antenna 190, which is located within cell 142 and connected to eNB 180 using any suitable connection means, such as a fibre optic cable.
For purposes of CoMP transmission and/or reception, some of the cells may form coordination areas. In the present example, cells 134, 140, and 142 form a coordination area 120, as indicated by the thick line in FIG. 1.
Furthermore, some or all of the eNBs within the wireless communications network 100 are connected by means of a transport network. This allows the eNBs to exchange information, e.g. for coordinating transmission or reception of signals in the cells being served by them. In FIG. 1, the transport network comprises the communication links 170 and 1100. It should be noted that it is not required to have a direct link between two eNBs for them to be able to exchange information over the transport network. For instance, eNB 180 is indirectly connected to eNB 110 via eNB 150, and may thus exchange information with eNB 110.
There are basically two approaches to providing the transport connectivity among the eNBs of the system. In the first approach, the eNBs are connected via dedicated links, such as fiber, optical, or microwave connections. In that case, two eNBs can exchange data, i.e. the cells they serve can be part of the same coordination area, only if there is an existing physical connection between the eNBs. In any other case, if it is desired to coordinate the cells served by these eNBs, a new physical connection has to be established. Multi-hop type of connections may also be applicable, as indicated above, but require more advanced methods of coordination and also increase the latency of the transport network. In the second approach, the eNBs are connected via a switched transport network, such as Gigabit Ethernet or Gigabit Passive Optical Network (GPON). In this case, all eNBs are connected to the network by using dedicated links, and two cells can be part of the same group by correctly setting the switches of the transport network corresponding to the serving eNBs. Hence, in this approach the physical connections among the eNBs exist but have to be enabled.
Two general approaches to forming coordination areas can be distinguished:
1. The coordination areas are formed per mobile terminal. In other words, each mobile terminal, e.g. user equipment (UE) is associated with a group of cells, which may or may not have a different composition than the group of cells used by other mobile terminals in the same geographical area. The coordination area for a mobile terminal typically changes dynamically over time. This solution is used e.g. for soft handover in CDMA.2. The cells in the network are divided into a set of coordination areas according to some algorithm, and this division then applies to all mobile terminals within the network. In principle, the different coordination areas may be partly overlapping, but in a typical and simple case there is no overlap of cells belonging to several coordination areas. A mobile terminal is associated with one of the coordination areas based on its current location, and data transmission and reception to the mobile terminal is handled within this coordination area.
The present disclosure will focus on the second approach, which is considered to be more suited to handle fast and advanced forms of simultaneous transmission to and reception from multiple mobile terminals foreseen to be employed in future networks.
In order to get the most benefit out of the CoMP network, coordination areas should be formed such that cells in the same coordination area have a high degree of mutual dependency, also referred to as coupling, while the coupling between cells belonging to different coordination areas is low. As mentioned above, a high degree of coupling between two cells means that transmissions in one cell strongly influence ongoing transmissions in the other cell. In other words, strong coupling implies high inter-cell interference. The strong coupling may be experienced in either the total cell coverage area or in subsections of the cell coverage area.
In scenarios where deployment, environment, and traffic is homogenous, coordination areas may be formed by grouping cells together based on geographical distance—i.e. cells that are geographically close are included in the same coordination area. In heterogeneous scenarios, however, traffic distribution, cell size, the number of sectors per site, and the physical environment may vary. For instance, large buildings and hills may affect the coupling between cells. In such scenarios, the geographical distance between cells may no longer be an adequate indication of the level of inter-cell interference. For instance, if two geographically adjacent cells are separated by a large building, the level of inter-cell interference between these cells may be very low. Thus, in heterogeneous networks it may no longer be possible to form suitable coordination areas based on the geographical distance between cells only.
If the coordination areas are formed in a less than optimal way, i.e. if the factors discussed above are not appropriately taken into account, the cooperation between the cells within the coordination areas will be less efficient. This may have a negative effect on radio network characteristics such as signal strength, system throughput, or latency.
Thus, there is a need for methods for forming coordination areas in heterogeneous networks, in order to achieve efficient CoMP deployment and improved radio network characteristics.